Governments and businesses have long used shredders to destroysensitive documents. How easy is it to reassemble the pieces?

Almost every office has one - a document shredder and a bin filled withstrips of paper fit for the bottom of a birdcage.

But in war time, the shredded pages found in a captured bunker orcommand post could contain intelligence, if the thousands of piecescould be reassembled.

After Iranian students seized the US embassy in Tehran in 1979, theyspent years painstakingly reassembling the intelligence reports andoperational accounts shredded by the CIA officers who were the lastworkers captured.

Now, a team of computer programmers from California have developedsoftware they say shows that computers can, in theory, do most of thehard work.

It works by matching up individual shreds based on minuscule clues ineach shred - the contour of the tears, a barely-visible watermark, andtraces of writing, for instance - and can work incalculably faster thana human undertaking the same task.

It was the successful entry in a document shredder competition launchedthis autumn by the US military, in an attempt to encourage research onwhat is essentially a maths problem - how to assemble a puzzleefficiently.

In October, the Defense Advanced Research Projects Agency (Darpa), thePentagon's research arm, offered $50,000 (£31,961) to the first team toreassemble five shredded hand-written documents and answer the puzzlescontained in each of them.

"Any time you're in conflict or in war and you were to take over abuilding or a compound, it wouldn't be terribly surprising to have theenemy try to destroy or shred their documents," says Dan Kaufman,director of Darpa's information innovation office.

"How can we quickly put them together and get some value and try tosave some lives?"

A decent commercial shredder can reduce a sheet of paper to more than400 pieces. That yields a total of 1,276,800 possible two-piececombinations - for one single-sided sheet.

Most office documents are a lot longer, many are printed on both sides,and the bin containing the shreddings could hold the remnants ofhundreds of pages.

The last embassy workers captured by the Iranian students on 4 November1979 included a team of CIA officers who had locked themselves in avault in order to burn and shred sensitive embassy documents, saysMalcolm Byrne, deputy director of the National Security Archive, aresearch organisation at George Washington University.

"When those guys gave themselves up, they left all the stuff in therethinking, 'okay, we've done our job,'" he says.

Instead, the Iranians laid the shreds out on a floor and devised asophisticated procedure for numbering, indexing and reassembling theindividual shreds, using carpet-makers skilled at weaving 400 knots persquare-inch rugs, Mr Byrne says.

"Certainly it took a number of years for them to finish the process,"he says.

The security forces later published the reconstructed documents in bookform and sold copies all over Tehran, he says. And agents used theintelligence they gathered to identify and kill CIA collaborators.

The Darpa competition opened on 27 October, and more than 9,000 teamsentered from across the US.

Each of the five shredded documents were presented online inhigh-quality digital format. Some documents were more than a single pageand some had pieces missing.

The winning team was a group of California computer programmers led byOtavio Good, a former video game developer.

He and his partners developed software that analysed the digital imagesof the shredded documents, using a concept called computer vision.

"We get the computer to look at where the ink is on the page and theshape of tear on the page," says Good, 37.

To reconstruct the document, a human user clicks on an individual piecethat has been ingested into the software, then selects which side of thepiece to check for a match. The software then recommends possiblematches from the remaining unmatched pieces.

This continues until all the pieces have been matched up.

"The process was more about having a human verify what the computer wasrecommending," he says.

It took the team, called All Your Shreds Are Belong to US, about amonth to develop and revise the software, and he estimates they spentabout 600 man-hours on the programming and eventual solution of thepuzzles.

"We basically spent every hour outside of work, working on this for amonth," Good says.

"And we did it for the competition. If we were doing it for the moneywe would have come out a lot lower than we could have just doingcontract work."

Good says the software the team developed has little potential as anoff-the-shelf product for use by the world's militaries and intelligenceagencies.

"I would call what we did a proof of concept," he says. "We put thisstuff together very quickly... and it's very specifically tailored toeach puzzle."

The Darpa documents were far simpler and neater than in a real case.All the sheets were single-sided, all the pieces were laid right-side-upin an orderly fashion to make them easier to work with.

And the shreds were unmarred by, say, smoke, mud or blood, unlikepieces captured in the field.

Good said he approached the competition as a programming challenge andwas "neutral" about the fact he was using his skills potentially to aidspies and soldiers.

"What we've done here is we've set the bar for where the security'sat," he says.

"A lot of these shredders are maybe not as secure as you thought, andmaybe you should get a better shredder if you want these really andtruly not to be assembled."

A camera that can shoot around corners has been developed by US scientists.

The prototype uses an ultra-short high-intensity burst of laser light to illuminate a scene.

The device constructs a basic image of its surroundings - including objects hidden around the corner - by collecting the tiny amounts of light that bounce around the scene.

The Massachusetts Institute of Technology team believe it has uses in search and rescue and robot vision.

"It's like having x-ray vision without the x-rays," said Professor Ramesh Raskar, head of the Camera Culture group at the MIT Media Lab and one of the team behind the system.

"But we're going around the problem rather than going through it."

Professor Shree Nayar of Columbia University, an expert in light scattering and computer vision, was very complimentary about the work and said it was a new and "very interesting research direction".

"What is not entirely clear is what complexities of invisible scenes are computable at this point," he told BBC News.

"They have not yet shown recovery of an entire [real-world] scene, for instance."Flash trick

Professor Raskar said that when he started research on the camera three years ago, senior people told him it was "impossible".

However, working with several students, the idea is becoming a reality.

The heart of the room-sized camera is a femtosecond laser, a high-intensity light source which can fire ultra-short bursts of laser light that last just one quadrillionth of a second (that's 0.000000000000001 seconds).

The light sources are more commonly used by chemists to image reactions at the atomic or molecular scale.

For the femtosecond transient imaging system, as the camera is known, the laser is used to fire a pulse of light onto a scene.

The light particles scatter and reflect off all surfaces including the walls and the floor.

If there is a corner, some of the light will be reflected around it. It will then continue to bounce around the scene, reflecting off objects - or people - hidden around the bend.

Some of these particles will again be reflected back around the corner to the camera's sensor.

Here, the work is all about timing.

Following the initial pulse of laser light, its shutter remains closed to stop the precise sensors being overwhelmed with the first high-intensity reflections.

This method - known as "time-gating" - is commonly used by cameras in military surveillance aircraft to peer through dense foliage.

In these systems, the shutter remains closed until after the first reflections off the tops of the trees. It then opens to collect resections of hidden vehicles or machinery beneath the canopy.

Similarly, the experimental camera shutter opens once the first reflected light has passed, allowing it to mop up the ever-decreasing amounts of reflected light - or "echoes" as Prof Raskar calls them - from the scene.

Unlike a standard camera that just measures the intensity and position of the light particles as it hits the sensor, the experimental set up also measures the arrival time of the particles at each pixel.

This is the central idea used in so-called "time-of-flight cameras" or Lidar (Light Detection And Ranging) that can map objects in the "line of sight" of the camera.

Lidar is commonly used in military applications and has been put to use by Google's Street View cars to create 3D models of buildings.

Professor Raskar calls his set-up a "time-of-flight camera on steroids".

Both use the speed of light and the arrival time of each particle to calculate the so-called "path length" - or distance travelled - of the light.

To build a picture of a scene, the experimental set up must repeat the process of firing the laser and collecting the reflections several times. Each pulse is done at a slightly different point and takes just billionths of a second to complete.

"We need to do it at least a dozen times," said Professor Raskar. "But the more the better."

It then use complex algorithms - similar to those used in medical CAT scans - to construct a probable 3D model of the surrounding area - including objects that may be hidden around the corner.

"In the same way that a CAT scan can reveal what is inside the body by taking multiple photographs using an x-ray source in different positions, we can recover what is beyond the line of sight by shining the laser at different points on a reflective surface," he said.Look ahead

At the moment, the set-up only works in controlled laboratory conditions and can get confused by complex scenes.

"It looks like they are very far from handling regular scenes," said Prof Nayar.

In everyday situations, he said, the system may compute "multiple solutions" for an image, largely because it relied on such small amounts of light and it was therefore difficult to extrapolate the exact path of the particle as it bounced around a room.

"However, it's a very interesting first step," he said.

It would now be interesting to see how far the idea could be pushed, he added.

Professor Raskar said there are "lots of interesting things you can do with it.

"You could generate a map before you go into a dangerous place like a building fire, or a robotic car could use the system to compute the path it should take around a corner before it takes it."

However, he said, the team initially aim to use the system to build an advanced endoscope.

"It's an easy application to target," he said. "It's a nice, dark environment."

If the team get good results from their trials, he said, they could have a working endoscope prototype within two years.

"That would be something that is room-sized," he said. "Building something portable could take longer."

VANCOUVER - A young Asian man who managed to board an Air Canada flight in Hong Kong while elaborately disguised as an elderly Caucasian male is seeking refugee status in Vancouver in what the Canada Border Services Agency is calling an "unbelievable case of concealment."

The story of the as-yet-unnamed man, whose silicone-mask adventure was making headlines around the world Friday, emerged the previous night from CNN, which obtained a copy of an internal alert issued earlier in the week by the Canadian border agency.

That bulletin — complete with an eyebrow-raising sequential set of before-and-after photos — describes how the man boarded Flight AC018 in Hong Kong on Oct. 29 wearing glasses, a brown cardigan sweater, a leather flat cap and a remarkable silicone mask to make him look like an elderly gentleman.

The passenger was seen at the start of the flight as an "elderly Caucasian male who appeared to have young looking hands," the CBSA bulletin said. Later in the flight, however, "the subject attended the washroom and emerged an Asian looking male that appeared to be in his early 20s."

The bulletin, which was posted to the Internet by CNN, said border services officials believe the young man managed to get on the plane with nothing more than an Aeroplan card and a boarding pass that belonged to a 55-year-old American man — neither of which reflect dates of birth.

"As neither Aeroplan cards nor boarding passes reflect dates of birth, it may not have been difficult for the very elderly looking imposter to pass himself off as a 55-year-old man."

Air Canada confirmed to The Canadian Press on Friday that a passenger onboard flight AC018 had been met by border services officials in Vancouver.

"The matter is still under investigation by the CBSA," said spokesman John Reber. The airline would not comment further on the incident as it remains an open case, he added.

All passengers flying to Canada from Hong Kong undergo multiple security checks before arriving at the Air Canada gates, Reber added. Those checks include the Chinese government-run Hong Kong passport control.

The CBSA bulletin said the man made a claim for refugee status after he was escorted off the aircraft.

It said the man initially claimed to own one bag, but flight crew turned over two additional pieces of baggage thought to belong to him. One of them contained a "disguise kit" with a "silicone-type head and neck mask," a brown leather cap, glasses and a thin brown cardigan.

The bulletin said the man proceeded to put on the disguise for border services officers, who then noted that he "very much resembled" an elderly Caucasian man, and even mimicked the movements of an elderly person.

The bulletin, which carried the headline "Unbelievable Case of Concealment," said the man admitted to officials that he had boarded the flight with the mask on and had removed it several hours later.

Agency spokeswoman Jennifer Bourque told CNN an individual was intercepted trying to enter Canada under false pretences on board an Air Canada flight.

She said the "foreign national is currently in CBSA detention" and that he will appear at an Immigration and Refugee Board hearing.

An Enigma machine is any of a family of related electro-mechanical rotor machines used for the encryption and decryption of secret messages. The first Enigma was invented by German engineer Arthur Scherbius at the end of World War I.[1] This model and its variants were used commercially from the early 1920s, and adopted by military and government services of several countriesÃ¢â‚¬â€most notably by Nazi Germany before and during World War II.[2] A range of Enigma models were produced, but the German military model, the Wehrmacht Enigma, is the version most commonly discussed.

The machine has become well-known because, during World War II, British and American codebreakers were able, following pioneering Polish work, to decrypt a vast number of messages which had been enciphered using the Enigma. The intelligence gleaned from this source, codenamed ULTRA by the British, was a substantial aid to the Allied war effort. The exact influence of ULTRA on the course of the war is debated; an oft-repeated assessment is that decryption of German ciphers hastened the end of the European war by two years.[3][4][5]

Though the Enigma cipher had cryptographic weaknesses, in practice it was only in combination with other factors (procedural flaws, operator mistakes, occasional captured hardware and key tables, etc.) that those weaknesses allowed Allied cryptographers to cryptanalyze so many messages.[6]